Proceedings of the Third International Conference on Invasive ...

<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaChapter 1: Spartina BiologyEFFECTS OF SALINITY ON PHOTOSYNTHESIS IN C 4 ESTUARINE GRASSESB.R. MARICLE 1,2 ,O.KIIRATS 1 ,R.W.LEE 1 , AND G.E. EDWARDS 11 School <strong>of</strong> Biological Sciences, Washington State University, Pullman, WA 99164-42362 Present address: Department <strong>of</strong> Biological Sciences, Fort Hays State University, Hays, KS 67601-4099; brmaricle@fhsu.eduThe effects <strong>of</strong> salinity on gross and net photosyn<strong>the</strong>sis rates were measured in estuarine C 4 grasses.Net CO 2 fixation was most affected by increasing salinity in Spartina patens and S. alterniflora,moderately affected in Distichlis spicata and S. densiflora, and unaffected in S. anglica. Spartinaanglica exhibited a decrease in internal carbon dioxide (CO 2 ) with increasing salinity, suggestingsome decrease in stomatal conductance. The results suggest S. anglica has a superior level <strong>of</strong> salttolerance compared to o<strong>the</strong>r species in <strong>the</strong> study, which could have implications for communitystructure in field sites or for invasive potential <strong>of</strong> S. anglica. The maximum quantum efficiency <strong>of</strong>CO 2 fixation, measured under limiting light, decreased with increasing salinity in S. alterniflora andS. patens, indicating an increase in leakage <strong>of</strong> CO 2 from <strong>the</strong> CO 2 pump. While carbon fixation decreasedunder increasing salinity in most species, fluorescence yield data showed <strong>the</strong>re was little effecton <strong>the</strong> use <strong>of</strong> solar energy in photochemistry. This indicates additional sinks are induced undersalinity for use <strong>of</strong> photochemically generated energy (e.g., increase in <strong>the</strong> CO 2 pump, photorespiration,or Mehler reaction). Also, in S. patens and D. spicata, which had moderate decreases in photosyn<strong>the</strong>sis,non-photochemical quenching (NPQ) mechanisms increased with salinity indicating some<strong>of</strong> <strong>the</strong> excess light energy was lost as heat. Therefore, excess excitation energy was diverted awayfrom <strong>the</strong> photosyn<strong>the</strong>tic reaction centers to prevent photoinhibition. Variable to maximal fluorescence(F V /F M ) ratios were not significantly decreased by increasing salinity, suggesting <strong>the</strong>re was nodamage to photosystem II (PSII) reaction centers in any species.Keywords: Spartina, Distichlis, chlorophyll fluorescence, gas exchange, salt stressINTRODUCTIONHigh and fluctuating salinity levels are characteristic <strong>of</strong>salt marshes. Elevated soil salinity can cause low waterpotentials, which can decrease stomatal conductance,reducing incoming CO 2 , and thus reducing photosyn<strong>the</strong>ticrates (Willmer 1983). One aim <strong>of</strong> <strong>the</strong> present study was todetermine how stomatal conductance and photosyn<strong>the</strong>sis inC 4 salt marsh grasses are affected by environmental salinity.Gross and net photosyn<strong>the</strong>sis rates were measured underthree salinity levels in <strong>the</strong> estuarine C 4 grasses Spartinaalterniflora, S. anglica, S. patens, S. densiflora, andDistichlis spicata in growth chamber studies. Differences in<strong>the</strong>se parameters were anticipated as a result <strong>of</strong> changes inphotosyn<strong>the</strong>sis as affected by salinity. Primary productionby Spartina is an important input into <strong>the</strong> estuarine system(Peterson et al. 1985), so understanding <strong>the</strong> photosyn<strong>the</strong>sisand hence production <strong>of</strong> this material may be important forunderstanding estuarine ecology and trophic interactions.MATERIALS AND METHODSPlants were collected at field sites in Washington (S.alterniflora, S. anglica, and D. spicata), Florida (S. patens),and Spain (S. densiflora). Tillers were potted in a 50/50volume to volume (v/v) sand/potting soil mixture, and plantswere watered to saturation twice weekly with modifiedHoagland solution (Epstein 1972). Growth chamberconditions were 14L/10D (light/dark hours) with 26°C daysand 18°C nights. Photosyn<strong>the</strong>tic photon flux density (PPFD)was 300 micromoles per square meter per second (μmol m -2s -1 ) at bench level. Flooded treatment plants were placed inlarge plastic tubs in a randomized block design. Water wasmaintained at 2 centimeters (cm) above <strong>the</strong> soil surface andwas completely replaced weekly. Salinity levels wereincreased 15 parts per thousand (‰) per week until floodedtreatments included 0, 15, and 30‰ salt (Instant Ocean salts;Aquarium Systems, Mentor, Ohio). Drained treatmentscontained 0‰ salt. Plants were held for 30 days under finaltreatment conditions before testing. There were at least threereplicate plants per species/treatment combination.Chlorophyll fluorescence was measured with an OS-500modulated fluorometer (Opti-Sciences, Inc.; Tyngsboro,MA). Gross photosyn<strong>the</strong>tic rates <strong>of</strong> O 2 evolution werecalculated from fluorescence yield measurements after Kralland Edwards (1992). The second-youngest leaf on each plantwas tested and light-response curves were generated forPPFD <strong>of</strong> 15-2000 μmol m -2 s -1 .A FastEst gas exchange system (Maricle et al. 2007) wasused to measure leaf gas exchange on high (30‰) and low(0‰) salinity plants. Intact leaves were enclosed in a leafchamber at 25°C and 25% relative humidity. Measures <strong>of</strong> CO 2uptake gave net photosyn<strong>the</strong>sis rates, and measures <strong>of</strong> externalwater vapor allowed calculations <strong>of</strong> stomatal conductance and-55-